Stents are used for a variety of medical purposes in the body including in the coronary arteries, the peripheral arteries, arteries of the neck, cerebral arteries, veins, biliary ducts, urethras, ureters, fallopian tubes, bronchial tubes, the trachea, the esophagus and the prostate. Stents are typically placed or implanted within a bodily vessel, for example, for treating stenoses, strictures or aneurysms therein. They are implanted to reinforce collapsing, partially occluded, weakened, or dilated sections of a blood vessel.
Stents may be self-expanding, mechanically expandable or hybrids. Examples of self-expanding stents include coil stents and stents made from shape memory materials such as nitinol. One such stent is disclosed in copending, commonly assigned U.S. application Ser. No. 08/511,076. Mechanically expandable stents are most often expanded by medical balloons. Such stents are typically made of metals such as stainless steel. An example of the latter is disclosed in U.S. Pat. No. 6,033,433. Hybrid stents may be mechanically expandable in part and self-expanding in part. An example of such a stent is disclosed in copending, commonly assigned U.S. Pat. No. 6,168,621.
Many of the stents known in the art have a tendency to shorten to varying degrees upon radial expansion of the stent. Braided stents, for example, tend to shorten axially upon radial expansion to a greater extent than many of the tubular stents having serpentine structures. As shown in FIG. 1, serpentine segment with struts 20 is shown having a unexpanded length Lu slightly longer than the expanded length Le of the segment whereas braided segment with struts 20 is shown having a unexpanded length Lu much longer than the expanded length Le of the segment.
Conversely, braided stents, upon the application of a compressive force thereto have a tendency to lengthen to a much greater extent than tubular stents having serpentine structures.
When a tubular stent with a serpentine structure is subjected to a compressive force, resistance to the compressive force will come largely from the radial direction. Typical methods of increasing the resistance to compressive forces involve increasing the width of the struts of the stent or the wall thickness. The use of wider struts or thicker walls may lead to less flexible stent designs or increases in the profile of the stent.
Unlike a tubular stent having a serpentine structure, when a braided stent is subjected to a compressive force, resistance to the compressive force will come mainly from the axial direction as a result of the greater axial forces transmitted to the vessel wall as the stent attempts to elongate.
There remains a need for stents which are capable of providing adequate support and which are capable of being subjected to significant compressive forces. This need is particularly felt in stenting arteries of the neck and other vessel which are more likely to be subjected to compressive forces.
Without limiting the scope of the invention, a brief summary of the invention is set forth below. Additional details of the summarized embodiments of the invention and/or additional embodiments of the invention may be found in the Detailed Description of the Invention below. A brief abstract of certain aspects of the technical disclosure in the specification is provided as well for the purposes of complying with 37 C.F.R. 1.72.
All US patents and applications and all other published documents mentioned anywhere in this application are incorporated herein by reference in their entirety.
In one embodiment, the invention is directed to a stent comprising a first portion which does not substantially lengthen axially upon radial contraction of the first portion and a second portion which lengthens axially upon radial contraction of the second portion. The second portion extends from the first portion and constitutes a free end portion of the stent.
The stent may optionally comprise a third portion which expands axially upon radial contraction of the stent. The third portion, where present, constitutes another free end portion of the stent.
Typically, the first portion is in the form of a tube with openings therein. The tube with openings therein may be made via any suitable technique including by cutting openings in an already existing tube or cutting openings in a sheet and rolling the sheet into a tube. The second portion is typically braided. Where present, the third portion will also typically be braided.
Desirably, the first portion of the stent will have a delivery configuration and an expanded configuration and will foreshorten by less than 10% upon expansion from the delivery configuration to the expanded configuration. Also desirably, the second portion will foreshorten axially by an amount in excess of 10% upon expansion from the delivery configuration to the expanded configuration.
The inventive stents disclosed herein may be made of a single piece of material or of a plurality of pieces of material.
The invention is also directed to a stent having a braided first free end portion and a non-braided portion adjacent thereto. Typically, the non-braided portion will be in the form of a tube having openings therein and may be formed as described above. The braided portion may have any number of braided strands. Desirably, the braided portion comprises no more than 10 braided strands. More desirably, the braided portion comprises no more than 8 braided strands.
The braided portion and the non-braided portion may be made of one piece construction or may be joined together via any suitable technique including welding and the use of adhesives.
The inventive stent may further comprise a braided second free end portion adjacent to the non-braided portion.
The invention is further directed to a non-braided outer stent having at least one braided inner portion joined thereto. In one embodiment, the stent has a single braided inner portion. In another embodiment, the stent comprises at least two braided inner portions, including one braided inner portion at a first end of the stent and another braided inner portion at a second end of the stent. Desirably, one of the braided inner portions extends outward in an axial direction from one end of the non-braided outer stent and the other of the braided inner portions extends outward in an axial direction from another end of the non-braided outer stent.
In yet another embodiment, the invention is directed to a method of forming a stent comprising the steps of providing a tube having a plurality of strands extending therefrom, and forming a braided portion with a flowpath therethrough by braiding the strands. Desirably, the braided portion comprises no more than 10 braided strands and more desirably, no more than 8 braided strands.
The inventive method may further comprise the steps of providing a tube and removing material from at least one end of the tube to form a plurality of strands extending from the tube.
In yet another embodiment, the invention is directed to a method of stenting a peripheral vessel comprising the steps of providing any of the inventive stents disclosed herein, delivering the stent to a desired location in a peripheral vessel and radially expanding the stent at the desired location in the peripheral vessel.
The invention is also directed to stents made in accordance with any of the inventive methods disclosed herein.
Additional details and/or embodiments of the invention are discussed below.